The present invention relates to a system and method for rapidly and confidently estimating the weights of mailpieces. More particularly it relates to weighing systems used in postage metering systems used to weigh and determine postage for mailpieces.
Postal scale systems are well known. Such scale systems weigh a mail piece and determine the appropriate postage for that mail piece as a function of the weight. Postal mailing systems where a mail piece is transported onto a postage scale system, the appropriate postage is determined, and the mail piece is then transported to postage metering system for imprinting with a postal indicium representative of the postage determined are also known. One such system is described in U.S. Pat. No. 4,742,878; issued May 10, 1988. In such systems there is a constant need to increase the rate at which the scale can determine the weight of a mailpiece in order that the throughput of the system can be increased.
Heretofore mailpiece weights typically have been determined by measuring the response of spring or other resilient member when the weight of a mailpiece is applied. Such systems have been successful but suffer from the inherent disadvantage that all such systems have a transitory initial response to the application of weight; and time must be allowed for the initial response to damp out and the system to reach, or at least approach, a steady state response, which is proportional to the mailpiece weight. Further such systems typically require a platform or support lager enough to support the largest mailpiece anticipated; and, where a sequence of mailpieces is to be transported through the system for processing, it is frequently necessary to halt each mailpiece to obtain an accurate weight measurement, thus further slowing or complicating the transport process.
An alternative approach that has been considered is sometimes referred to as to as dimensional weighing; where a mailpiece weight is estimated by measuring the mailpiece volume and multiplying the volume by a predetermined density for paper (preferably taken to be approximately 0.54 ounces/cubic inch). It is believed that a dimensional weighing system would be, relative to typical weighing systems, both faster, since dimensional measurements can easily be made as the mailpieces are transported at a relatively high continuous speed, and less expensive, since the various complex measures taken to reduce the time required to obtain an accurate weight are not needed. Dimensional weighing systems also have the advantage that dimensional measurements can be made as a mailpiece is transported transversely to a cross sectional plane so that the need for an extended platform to support the mailpiece as the weight is determined is eliminated and the system footprint is reduced; nor is it necessary to stop each mailpiece to determine its weight. Dimensional weighing systems have not, however, proven successful because of a low confidence in their accuracy, as materials other than paper incorporated in a mailpiece can cause large deviations between the actual weight and the weight estimated by dimensional weighing.
Thus it is an object of the subject invention to provide a dimensional weighing system that provides weight estimates which can be confidently accepted.